This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Near-Infrared Tissue Optics The red to near-infrared (NIR) part of the electromagnetic spectrum (600 to 1000 nm) allows photons to penetrate a few centimeters below the surface of the skin.(1-3) These photons are non-ionizing and do not induce local heating. Quantitative optical spectroscopy in the NIR allows for safe, non-invasive measurements of the concentrations of blood, water, and lipids in tissues.(2, 4-6) Diffuse Optical Spectroscopy (DOS) has been in use for clinical studies in the Tromberg lab for over 8 years in breast (UCI protocols #95-563, #99-2183, #02-2306), adult muscle (#01-1924) and neonatal muscle (#01-2011). These studies have clearly shown the enormous sensitivity of DOS to hemodynamic events in tissues.(2, 7-12) The pulse oximeter is a similar commercial instrument that has found wide clinical acceptance. This optical device uses two wavelengths, one red and the other near-infrared, to measure the arterial hemoglobin saturation (SaO2) of blood. Although the risks are essentially the same, DOS measurements offers information not provided by pulse oximetry(13): (1) DOS measurements are able to measure the absolute concentrations of reduced and oxygenated hemoglobins ([Hb-R] and [HbO2], respectively) as well as the water and lipid concentrations of the tissue. These hemodynamic parameters have been shown to correlate with invasive measures of cardiac output, mean arterial pressure, blood loss, and blood hemoglobin concentration.(11) (2) Pulse oximetry only measures the oxygen saturation of the arterial blood (SaO2). DOS samples the blood of the tissue microvasculature.(14) Thus the DOS signal is more representative of the perfusion of blood and consumption of oxygen in the muscle. (3) Pulse oximetry may only be used in regions of thin tissue whereas DOS measurements probe the underlying tissue up to a few centimeters deep, and can measure hemoglobin concentrations in tissues such as muscles rather than at the skin surface. Such measurements provide information about a region of tissue. We and other research groups have proposed to use DOS as a means to monitor muscle physiology, for example during exercise or in response to external factors such as shock.(11, 15-24) The sensitivity of DOS to tissue hemodynamics provides an impressive arena of useful clinical applications. But very little is known about the normal optical properties of muscle tissue in vivo. Since NIR optical properties are directly related to tissue physiology, it is known that physiological changes will manifest as optical property changes. This is obvious in the case of exercise, but the degree to which muscle optical properties will change due to normal physiological variations is unknown. It is unclear how much these variations affect standard measurements, and if they contain any novel information. Our success with measuring spectroscopic information from breast cancer tumors has led us to believe that there may be important spectroscopic signatures in muscle tissues that signify disease processes that have yet to be discovered. For example, it is known that in cachexia (decay of tissues, which is typical in some types of cancer or other chronic disease) reduces muscle mass due to unknown alterations to tissue metabolism and function. Although it is beyond the scope of this trial to perform a detailed investigation of metabolic disorders such as cachexia, it is important to study and characterize both normal and diseased tissues.